The 5:2 Fast Diet explained

No one likes the thought of starting a diet; they’re restrictive and boring, and it usually means you can forget about reaching for the chocolate.

But what if you could go on a diet that involves eating exactly what you like? On the 'Fast Diet' you can do just that - eat normally for five days a week and fast for two. It’s been described as ‘ground-breaking’, and has attracted a global following, including Miranda Kerr and many Essential Baby forum members. But is it really as simple as it sounds?

Dr Michael Mosley, author of the international best selling book The Fast Diet, certainly thinks so. “You eat normally five days a week, then for just two days a week you cut your calories (500 for women, 600 for men),” he said.

If you stick to the plan he says you can expect to lose 0.5kg a week, but the benefits don’t stop there. Two scientific studies have found that eating less for one or two days a week acts as a breast cancer preventative, and reduces markers for heart disease and insulin sensitivity (associated with diabetes). Sounds great, but how does it work?

The claim is that the origin of the diet lies with our remote ancestors, who evolved to suit periods of both feast and famine. When the tribe made a kill, everyone ate well, but when the hunt was less successful, everyone went hungry. To survive during times of famine, the body switches into ‘repair’ mode, aiming to stay in good shape until food is available again.

“Intermittent fasting can put us back in touch with our human selves. It is a route not only to weight loss, but also to long-term health and wellbeing,” said Dr Mosley.

Pip Golley, accredited practicing dietitian and spokesperson for the Dietitians Association of Australia (DAA), agrees that there is some evidence that the Fast Diet is an effective way to lose weight.

“Evidence is still in its early stages, but if managed well it looks like there are some benefits, including weight loss,” she said.

"However, it is not a diet that is recommended by the DAA”.

Golley warns that while the Fast Diet will work well for some people, there are others who should avoid it.

“It’s not suitable for people with mental health issues – including postnatal depression or disordered eating – or woman who are pregnant or breastfeeding,” she said.

Although fasting is unlikely to affect a woman’s ability to produce breast milk, it’s important to remember that breastfeeding places extra demands on the body. On average, women who are breastfeeding need an extra 400-500 calories a day, along with increased nutrient needs.

“A nutrient-dense diet is very important for the breastfeeding period”, said Golley.

‘It works for me’

While the Fast Diet isn’t suitable for breastfeeding or sleep-deprived mothers, it has worked well for others. For Gina Baynham, mother of three, the Fast Diet has proved to be successful: she’s lost 8kg in five months.

“The first few weeks were quite hard, but once I discovered that I could cope with the reduced food intake on the fast days it became much easier. And it’s only two days a week – on my non-fast days I can indulge a little in ‘treat’ food,” she says.

On her fast days, Baynham uses 150 calories for a small breakfast (typically things like porridge or a small omelette), and 300 calories for a light dinner (stir-fries and salads are her favourites). Another 50 calories are saved for a small jug of skimmed milk so she can still enjoy tea and coffee during the day.

But the diet is flexible; if you prefer to eat regular, small meals of 80–100 calories throughout the day, it will still have a benefit.

The Fast Diet has been so successful for Baynham that she intends to continue with it long term. “I feel I have the magic formula to consistent healthy weight and I cannot imagine why I would ever stop,” she said.

The fine print

However, Professor Tim Gill, associate professor and principal research fellow at the Boden Institute of Obesity, Nutrition, Exercise and Eating Disorders at the University of Sydney, told the Sydney Morning Herald that positive results from the diet are due to simply reducing overall total calorific intake, as with most diets.

''There's no magic, no suggestion of any physiological advantage. The theory is that if you reduce intake constantly then the body adapts, but for 12 or 16 hours in the day we aren't eating anyway,” he said.

“I don't think it is a panacea but it is a way of structuring calorie reduction for some people … If every diet worked for everyone we would only ever have had one diet.''

Professor Amanda Sainsbury-Salis, also from the university's Boden Institute, said more research was needed on the best time-reduced calorie periods. She also said it was important to have medical supervision for severe calorie reduction, as it can result in some problems, such as gall stones.

Professor Sainsbury-Salis also made clear that it's not a fad diet, but a new movement in weight management.